Solar cells are photovoltaic (PV) semiconductor devices that convert sunlight into electricity. Photons from the sunlight knock electrons loose from their atoms, generating a current through the material. These photovoltaic semiconductors may be made from single crystal silicon (mono-crystalline), polycrystalline (multi-crystalline) silicon, amorphous silicon or other compound semiconductor materials. In simple terms an electrical P-N junction is created in PV material or substrate i.e. silicon wafer. This material is then connected to an electrical circuit by means of the positive and negative electrode. When exposed to sunlight a current is generated in the PV material due to a photoelectric effect.
There are various processing methods used to manufacture a PV solar cell, and several manufacturing process steps are required. Of particular interest for this invention are the etching steps, where a controlled amount of material has to be removed from the substrate and deposited layers; including wafer damage removal, cleaning, texturing, polishing, and by-product (PSG) removal.
Texturing of the substrate material; more particularly the control of this texturing is difficult to achieve. Different textures have been shown to have different enhancing properties for different solar cell substrates and designs. In most solar cell designs, when a light trapping pattern is textured onto the starting substrate on the sunny side of the wafer, an increase in cell efficiency is observed. Texturing the surface of the substrate will enlarge the surface area and lower the reflectivity of the base material therefore capturing more of the light that falls on the surface. However the texture needs to be carefully controlled and optimised so as integrate well with subsequent steps in the solar cell manufacturing process. The textures that are formed need to be easily doped and passivated and must not damage the carrier lifetimes of the base silicon substrate. These demands on the texture for PV solar cell manufacturing require a much higher level of control over the process than has been previously available. In the prior art, for example, the use of wet chemicals etching cannot deliver optimised textures for different cell designs as the etching mechanism is defined by the crystallographic structure of the substrate to be etched. Also, true single-sided etching of a substrate is not offered. As an alternative to wet chemical etching some methods have been proposed, including reactive ion etching and other plasma based silicon etching techniques well known to semiconductor device manufacturing. These plasma based techniques can have a detrimental effect on the carrier lifetimes of the base silicon and in some cases do not lead to a texture that can be easily passivated.
Wet Chemical Etching is the current state of the art for PV solar cell manufacturing. Currently these steps are carried out via a wet chemical process where the silicon substrates are dipped in a heated Acidic or Alkaline solution for a period of time to form uneven patterns on the surface of the substrate.
The problems associated with wet chemical etching for crystalline solar cells production are:                1. Very large amounts of water and other chemicals are consumed during these wet chemical etch steps.        2. Wet etch process is not adequate for thinner wafers (<160 μm).        3. Texturing by the traditional alkaline wet chemical etching of the cheaper multi-crystalline silicon solar cell does not give satisfactory antireflective properties. This is due to anisotropic nature of the multi-crystalline silicon substrate.        4. The process of record for multi-crystalline silicon solar cell manufacturing is to use an additional wet etch step using an acidic solution e.g. a mix of HF and HNO3. This “Isotexturing” results in lower reflection than the traditional anisotropic etching on multi-crystalline substrates. However, as outlined in point 1, it does not give the same results as traditional alkaline etching on mono-crystalline substrates. This leaves the overall cell conversion efficiencies of multi-crystalline silicon solar cells to lag behind the conversion efficiencies of the more expensive mono-crystalline silicon solar cells.        5. The wet etch equipment can be very large (in same case up to 17 m long) and has limited process throughput capability.        6. Wet etch efficiency is highly dependent the crystallographic structure of the wafer and requires different chemicals recipes for mono or multi-Si wafer.        7. Limited single sides of the wet process that does not provide for decoupled surface treatment.        8. Wet etch needs some defects in the wafer to work. It does not work from a smooth surface.        
Dry etching plasma-less processes have been recently used for vacuum chamber cleaning application. For example, as described in US2008142046, molecular Fluorine (F2) was used for the cleaning of Silicon Nitride (SiNx) in a chemical vacuum deposition (CVD) chamber. Pre-diluted fluorine in an inert gas is delivered into the vacuum chamber and the fluorine is thermally dissociated by heating the chamber to a temperature between 230 to 565 deg C. The undesired silicon nitride is removed from the inner chamber's surfaces by chemical reaction. This vacuum chamber cleaning process is required in order to prevent cross-process contamination.
U.S. Pat. No. 6,500,356 (B2) describes a dry etching process that selectively etch Silicon on electronic devices, without etching Silicon oxide or silicon nitride. The specification is very much restricted to holding the substrate in the interior of a vacuum chamber and supplying the etch gas to the same chamber, at less than atmospheric pressure (260 millitorr (mT)).
Dry etching of PV solar cells has been under development for some time. There have been numerous publications using traditional vacuum based plasma etching techniques. The main problem with these methods is the PV material is damaged caused by the ions from the plasma during the etching process. Although some progress has been made in regard to this plasma damage issue, the commercialisation of such solutions is unlikely as the cost of the vacuum-based systems is prohibitive and the low throughput is not suitable for large-scale solar PV cell manufacturing.
There has also been work done regarding atmospheric plasma used to treat, clean and, in some cases, modify the surface of various substrates. In EP0690479 a method utilising an atmospheric plasma discharge to treat substrates at atmospheric pressure is described. Similarly, US2008/0000497 A1 describes a method of removing organic-containing layers using a plasma discharge.
In US 2008/0305643 an atmospheric plasma etching apparatus to remove the doped surface layers on the back faces of a crystalline solar wafer is described. Similarly, US 2010/0062608 describes an apparatus to selectively etch the phosphorus silicate glass (PSG) formed during the diffusion process using a plasma-based process. All of this state of the art is based on work utilising plasma discharge technology and using the plasma to generate the active species required to do the work on the substrates.
U.S. Pat. No. 4,803,947 discloses an apparatus for deposition of thin film amorphous Si PV films and surface treatment of PV substrates utilising a non-plasma technique. U.S. Pat. No. 4,803,947 makes reference to the use of F2 to affect a texture or “coarsening” on the surface to aid light absorption. It also describes an array of chambers that are connected but the process regions are separated by purge gas curtains. This apparatus utilises a Roll-to-Roll method that is limited for use with flexible substrates that can be conveyed and collected on rollers. The use of the purge curtains is limited to separating process reaction spaces within a larger enclosed system that house all process chambers along with the delivery and collection roller chambers. Even with the capability of moving the substrates freely between process chambers, there is no continuous production. When the process is complete both the deployment and collection rolls would need to be changed and the system opened in some way that is not described. The coarsening effected by the use of F2 is not claimed to be controlled or optimised in any way. The flow levels and pressure described indicate that this process takes place in a vacuum i.e. 30 SCCM at 1.5 torr. However, this method is not suitable for processing wafers.
There is therefore a need to provide a process and apparatus for dry etching/texturing of crystalline silicon photovoltaic solar cell wafers and control, which overcomes the above-mentioned problems.